In the electrolytic refining or recovery of nonferrous metals such as copper, i.e. so-called winning and electro-deposition, the nonferrous metal is deposited from an electrolyte solution containing salts thereof upon a cathode which is spacedly juxtaposed with an anode in the electrolytic cell. Both the anode and the cathode are commonly oriented vertically and, for example, may be supported by hangers or the like at their upper ends. The cathode may be composed of the metal to be refined, or an inert material, while the anode may be an inert plate.
Because of the electrochemical nature of the deposition operation, various phenomena which are familiar in electrolyte systems and at electrode-electrolyte interfaces arise, including concentration gradients from top to bottom in the electrolyte, conductivity gradients, polarization, ion contamination etc.
Thus it has been recognized that some degree of relative movement of the electrode and the electrolyte of the cell is advantageous to improve the efficiency from the energy or metal-deposition viewpoint.
For example, nonferrous metals are usually for electrolytic recovery from electrolyte solution containing the metal in low concentrations. The circulation of the electrolyte can result in an equalization of the concentration over the entire surface of the anode and within the cell so that a depletion of the nonferrous metal in the cathode region does not result in premature evolution of hydrogen, thereby decreasing the electrical current efficiency and resulting in the formation of poor, nonhomogeneous (spony) deposits of metal.
Numerous techniques have been used to agitate or displace the electrolyte relative to the electrodes of the cells. For example, the electrolyte may be circulated or pumped past the electrode surface at greater or lesser speeds, may be induced to rise and lower by the application of gas pressure or displacement forces, may be agitated or stirred by stirring devices in contact with the electrolyte, and even may be displaced by movement of one or more electrodes. The simple movement of the electrodes may itself serve to bring about the relative displacement or combinations of these techniques may be employed.
For the most part, however, efforts have concentrated on high velocity pumping of the electrolyte between the electrodes (see Ullmann's Encycklopadie der technischen Chemie, 4th Edition, Vol. 3, p. 268; V. Tafel: Lehrbuch der Metallhuttenkunde, Vol. 1, p. 552, 1951; Die technische Elektrometallurgie wasseriger Losungen, Part I, Akademisch Verlagsgesellschaft Geest & Portig K.-G., p. 129, Leipzig, 1961).
A disadvantage of this system is that efficient relative displacement of the electrode and the electrolyte requires the creation of turbulence at the interface and, with the usual cell dimensions, the pumping of the electrolyte does not meet the criteria for developing such turbulence. Efforts to alter the cell dimensions to ensure turbulence have been found to diminish the access of the electrolyte to the electrodes. Consequently, the pumping technique by mass displacement of the electrolyte is not sufficiently effective and is not efficient for the purposes.
It has also been proposed to effect the relative displacement of the electrolyte and the electrodes and/or agitation of the electrolyte by so-called gas-lift techniques whereby a gas is introduced into the electrolyte at the bottom of the cell and gas bubbles rise in the electrolyte for agitation and entrainment purposes.
For example, in British Pat. No. 1,392,705, a system is described in which the gas is discharged from a pipe system disposed at the bottom of the cell. U.S. Pat. No. 3,959,112 describes a similar arrangement in which the gas discharge pipes are porous so that the bubbles rise in a fine curtain. Other systems in which pipes are mounted or provided at the bottom of the cell are found in U.S. Pat. 3,928,152 and German patent document (Open Application--Offenlegungsschrift) No. 2,508,094.
While upwardly moving gas bubbles are effective to bring about the desired agitation of the electrolyte, problems have been encountered with the earlier systems described. For example, it is difficult, because the pipe systems are located at the bottom of the cell, to readily clean the latter. Furthermore, the air-bubbling arrangements tend to become encrusted or covered by cell precipitates or sediment during cell operation, the sediments or encrustations tending to block the introduction of air into the electrolyte.